Navien NFB Series User manual

WARNING
If the information in these instructions is not followed exactly, a fire or explosion may result, causing
property damage, personal injury, or death.
Do not store or use gasoline or other flammable vapors and liquids in the vicinity of this or any other
appliance.
WHAT TO DO IF YOU SMELL GAS
●Do not try to light any appliance.
●Do not touch any electrical switch; do not use any phone in your building.
●Immediately call your gas supplier from a neighbor’s phone. Follow the gas supplier’s instructions.
●If you cannot reach your gas supplier, call the fire department.
Installation and service must be performed by a qualified installer, service agency or the gas
supplier.
The installation must conform with local codes or, in the absence of local codes, the National Fuel
Gas Code, ANSI Z223.1/NFPA 54 and/or CSA B149.1, Natural Gas and Propane Installation Code.
Keep this manual near this boiler for future
reference whenever maintenance or service
is required.
Installation & Operation Manual
NFB Condensing Boilers
Model
NFB-175
NFB-200

22
Requirements for the State of Massachusetts
NOTICE BEFORE INSTALLATION
This appliance must be installed by a licensed plumber or gas fitter in accordance with the Massachusetts Plumbing and Fuel Gas Code 248
CMR Sections 4.00 and 5.00.
IMPORTANT: In the State of Massachusetts (248 CMR 4.00 & 5.00)
For all side wall horizontally vented gas fueled equipment installed in every dwelling, building or structure used in whole or in part for
residential purposes, including those owned or operated by the Commonwealth and where the side wall exhaust vent termination is less than
seven (7) feet above finished grade in the area of the venting, including but not limited to decks and porches, the following requirements shall
be satisfied:
1. INSTALLATION OF CARBON MONOXIDE DETECTORS. No installation or replacement of a vented gas appliance shall be permitted unless a
battery powered or electrically hard wired carbon monoxide detector is present on the same floor as the appliance or on the next adjacent
floor when the appliance is located in a crawl space unless the appliance is located in a detached, uninhabitable garage. For all residential
dwellings, a carbon monoxide detector must also be present on each habitable level of the dwelling. These requirements shall not be
deemed to waive any additional requirements imposed by M.G.L. c. 148 §26F1/2.
2. APPROVED CARBON MONOXIDE DETECTORS. Each carbon monoxide detector as required in accordance with the above provisions shall
comply with NFPA 720 and be ANSI/UL 2034 listed and IAS certified.
3. SIGNAGE. Whenever any through-the-wall vent is installed less than seven feet above the finished grade, a metal or plastic identification
plate shall be permanently mounted to the exterior of the building at a minimum height of eight feet above grade directly in line with the
exhaust vent terminal. The sign shall read, in print size no less than 0.5 inches in size,“GAS VENT DIRECTLY BELOW. KEEP CLEAR OF ALL
OBSTRUCTIONS”.
4. INSPECTION. The state or local gas inspector of the side wall horizontally vented gas fueled equipment shall not approve the installation
unless, upon inspection, the inspector observes carbon monoxide detectors and signage installed in accordance with the provisions of
248 CMR 5.09 and 5.12.

3Contents
Contents
1. About the Boiler 6
1.1 Included Items 6
1.2 Accessories 6
1.3 Specifications 7
1.4 Components 9
1.5 Dimensions 11
1.6 Rating Plate 12
2. Installing the Boiler 13
2.1 Removing the Wood Pallet from the boiler 13
2.2 Choosing an Installation Location 14
2.3 Mounting the Boiler to the Wall 16
3. Installing the System Piping 17
3.1 Installing a Space Heating System 17
3.2 Connecting the Condensate Drain 24
3.3 Filling the System 27
3.4 Testing the Water System 27
3.5 Examples of System Applications 28
3.6 Examples of Electrical Connections 34
4. Connecting the Gas Supply 46
4.1 Gas Pipe Sizing Tables 48
4.2 Measuring the Inlet Gas Pressure 49
5. Venting the Boiler 51
5.1 Selecting a Vent Type 52
5.2 Selecting Vent Pipe Materials 55
5.3 Measuring Vent Length 56
5.4 Terminating the Vent 56
6. Setting the DIP Switches 59
6.1 DIP Switch 1 (6 switch unit) 59
6.2 Dip Switch 2 (8 switch unit) 59
7. Connecting the Power Supply 60
8. Installing a Cascade System 61
8.1 Connecting Water Supplies 61
8.2 Connecting the Communication Cables 65
9. Installing a Common Vent System 66
9.1 About the Common Vent System 66
9.2 Navien Backflow Damper (Back-draft Damper) 71
9.3 Starting the Common Vent System 72
9.4 Connecting and Terminating the Vent Pipe 74
9.5 Setting Up the Common Vent System 75
9.6 Common Vent Termination Clearances 75
9.7 Installing a Condensate Drain 77
9.8 Exhaust Vent Termination Clearances (For Direct Vent) 78
9.9 Maintenance
79
10. Installation Checklist 80
11. Operating the Boiler 83
11.1 Turning the Boiler On or Off 83
11.2 Adjusting the Temperature 83
11.3 Accessing Basic Menu Items 85
11.4 Accessing Advanced Menu Items 89
12. Appendixes 97
12.1 Gas Conversion 97
12.2 Wiring Diagram 102
12.3 Ladder Diagram 103
12.4 Outdoor Temperature Sensor 104
12.5 Outdoor Reset Control 105
12.6 Component Assembly Diagrams and Parts Lists 106

4Safety Information
The following safety symbols are used in this manual. Read and
follow all safety instructions in this manual precisely to avoid
unsafe operating conditions, fire, explosion, property damage,
personal injury, or death.
DANGER
Indicates an imminently hazardous situation which, if not
avoided, could result in severe injury or death.
WARNING
Indicates a potentially hazardous situation which, if not
avoided, could result in injury or death.
CAUTION
Indicates a potentially hazardous situation that, if not avoided,
could result in property damage.
DANGER
If you smell gas:
●Do not try to light any appliance.
●Do not touch any electrical switches or use landline phones.
●From a neighbor’s phone, call your gas provider and follow
their instructions.
●If you cannot reach your gas provider, call the fire
department.
●Do not return to your home until authorized by your gas
supplier or the fire department.
Do not use or store flammable products, such as gasoline,
solvents, or adhesives in the same room or area as the
boiler.
●The boiler has a main burner flame that can turn on at
any time and can ignite flammable vapors. Vapors from
flammable liquids can explode and catch fire, causing death
or severe burns.
●Vapors cannot be seen and are heavier than air. They can
travel long distances along the ground and can be carried
from other rooms to the boiler’s main burner flame by air
current.
●Keep all flammable products far away from the boiler and
store them in approved containers. Keep the containers
closed tightly and out of the reach of children and pets.
Safety Information

5Safety Information
CAUTION
●Do not turn on the boiler unless the water and gas
supplies are fully opened.
Doing so may damage the boiler.
●Do not use this boiler for anything other than its
intended purpose, as described in this manual.
●Do not remove the front cover unless the power to the
boiler is turned off or disconnected.
Failure to do so may result in electric shock.
●When servicing the controls, label all wires prior to
disconnecting them.
Failure to do so may result in wiring errors, which can lead to
improper or dangerous operation. Verify proper operation
after servicing.
●Do not use unapproved replacement or accessory parts.
Doing so may result in improper or dangerous operation
and will void the manufacturer’s warranty.
●Do not place anything in or around the vent terminals,
such as a clothes line, that could obstruct the air flow in
or out of the boiler.
●This boiler has been approved for use in the USA and
Canada only.
Using the boiler in any other country will void the
manufacturer’s warranty.
WARNING
●Do not store or use gasoline or other flammable liquids
near this boiler.
Doing so may result in fire or explosion.
●Do not place combustibles, such as newspapers or
laundry, near the boiler or venting system.
Doing so may result in a fire.
●Do not place or use hair sprays, spray paints, or any other
compressed gases near the boiler or venting system,
including the vent termination.
Doing so may result in fire or explosion.
●Do not operate the boiler with the front cover opened.
Doing so may result in fire or carbon monoxide (CO)
poisoning, which may result in property damage, personal
injury, or death.
●Do not operate this boiler without proper venting.
Doing so may result in fire or carbon monoxide (CO)
poisoning, which may result in property damage, personal
injury, or death.
●Do not touch the power cord or internal components of
the boiler with wet hands.
Doing so may result in electric shock.
California law requires the following Prop 65 warning to be
provided:
WARNING
This product can expose you to chemicals including lead, lead
compounds, and carbon bisulfide which are known to the
State of California to cause cancer and birth defects or other
reproductive harm. For more information, go to
www.P65Warnings.ca.gov.

6About the Boiler
1.1 Included Items
When you open the box, you will find the following items with
the boiler. Check the box for each of the following items before
installing the boiler.
Installation & Operation Manual,
User’s Information Manual Quick Installation Manual
2 in Vent Termination Caps (x2) 2 in Wall Flanges (x4)
Air Vent Wall Mounting Bracket
Tapping Screws & Anchors (x4) Pressure Relief Valve, Heating
Conversion Kit Outdoor Temperature Sensor and
Cable
Air Vent Bushing (3/4in to 1/2in) Condensate Trap
System Temperature Sensor
1. About the Boiler
1.2 Accessories
The following optional accessories are available for the boiler.
NFB-175/200 Manifold System
(for single boilers) Condensate Neutralizer Kit
Zone Controller
(FMZ-40/60)
Zone Controller
(FMZ-20/30)
Universal Temperature Sensor
(with clip)
Navi-Link
(Wi-Fi Control System)
Ready-Link Rack NaviClean
Note The Universal Temperature Sensor can be used as a
System Temperature Sensor or a DHW Tank Sensor.

7About the Boiler
1.3 Specifications
The following tables list the specifications for the boiler. Additional specifications for water, gas, electricity, and vent connections are provided
in the Installation section.
Space Heating Specifications
Navien Condensing Boiler
Space Heating Ratings Other Specifications
Model Number
1
Heating Input (MBH) Heating
Capacity
2
(MBH)
Net AHRI
Rating Water
3
(MBH)
AFUE
2
(%) Water
Pressure
Water
Connection
Size (Supply,
Return)
Min Max
NFB-175 17.5 175 161 140 95 12–80 psi 1 ¼ in NPT
NFB-200 19.9 199 183 159 95
Note 1. Ratings are the same for natural gas models converted to propane use.
2. Based on U.S. Department of Energy (DOE) test procedures.
3. The net AHRI water ratings shown are based on a piping and pickup allowance of 1.15. Consult Navien before selecting a boiler
for installations having unusual piping and pickup requirements, such as intermittent system operation, extensive piping
system, etc.

8About the Boiler
General Specifications
Item NFB-175 NFB-200
Dimensions 17.3 in (W) x 17.3 in (D) x 27.6 in (H)
Weight 123 lb (56 kg)
Installation Type Indoor Wall-Hung
Venting Type Forced Draft Direct Vent
Ignition Electronic Ignition
Natural Gas Supply Pressure (from source) 3.5 in–10.5 in WC
Propane Gas Supply Pressure (from source) 8.0 in–13.5 in WC
Natural Gas Manifold Pressure -0.05 to -0.70 in WC -0.05 to -0.20 in WC
Propane Gas Manifold Pressure -0.10 to -0.68 in WC -0.10 to -0.45 in WC
Gas Connection Size 3/4 in NPT
Power Supply
Main Supply 120V AC, 60Hz
Maximum Power
Consumption Less than 15 amperes
Materials Casing Cold-rolled carbon steel
Heat Exchangers Stainless Steel
Venting
Exhaust
2 in or 3 in PVC, CPVC, approved polypropylene*
2 in or 3 in Special Gas Vent Type BH (Class III, A/B/C)
2 in or 3 in Stainless Steel
Intake
2 in or 3 in PVC, CPVC, polypropylene
2 in or 3 in Special Gas Vent Type BH (Class III, A/B/C)
2 in or 3 in Stainless Steel
Vent Clearance 0 in to combustibles
Safety Devices
Flame Rod, APS, Ignition Operation Detector
Water Temperature High Limit Switch, Exhaust Temperature High Limit Sensor, Water
Pressure Sensor
Temperature Setting Range
Item Temperature Setting Range Remarks
Space Heating
Supply 77–194°F (25-90°C) Actual supply and return temperatures vary
depending on the selected outdoor reset
curve.
Return 68–158°F (20-70°C)
Note For more information about the space heating temperature setting range, refer to“11.3.3 Setting the Space Heating Operation” on
page 83.

9About the Boiler
1.4 Components
The following diagram shows the key components of the boiler. Component assembly diagrams and particular parts lists are included in the
Appendixes.
Intake Air
Air Intake Pipe
Exhaust
Fan & Motor Assembly
Dual Venturi
APS
PCB
Power Switch
Air Vent Connection
Gas Pipe
Mixing Chamber
Igniter
Flame Rod
Fire-Tube Type
Heat Exchanger
ASME Name Plate
Front Panel
24 V Power Transformer
Gas Valve
Condensate Outlet
Gas Inlet
Condensate Trap
(Siphon)

10 About the Boiler
Terminal Block
Dual Venturi
Fan & Motor Assembly
Fire-Tube Type
Heat Exchanger
APS
Power Switch
Heating Return
Gas Valve
ASME Name Plate
Mixing Chamber
Flame Rod
Igniter
24 V Power Transformer
Heating Supply
Condensate Trap
(Siphon)
PCB
Front Panel
Gas Pipe
Air Intake Pipe

11About the Boiler
1.5 Dimensions
The following diagrams show the dimensions of the boiler and the table lists the supply connections.
27.6" (702 mm)
Supply Connections
Description Diameter
AAir Intake 2 in
BExhaust Gas Vent 2 in
CAir Vent Connection
3
/
4
in
DHeating Supply 1
1
/
4
in
EHeating Return 1
1
/
4
in
FGas Connection
3
/
4
in
GCondensate Outlet
1
/
2
in
Overhead View
AB
C
3.8” (97 mm)
4.3” (109 mm)
4.1” (105 mm)
9.7” (246 mm)
17.3” (440 mm)
Supply Connections
14.6” (370 mm)
2.6” (67 mm)
12.1” (308 mm)
6.3” (160.5 mm)
DE F G
17.3” (440 mm)
9.6” (244 mm)3.2” (81.7 mm)

12 About the Boiler
1.6 Rating Plate
The Navien NFB boilers come from the factory configured for use with Natural Gas (NG). Before starting the installation, check the rating
plate located on the side of the boiler to ensure that the boiler matches the gas type, gas pressure, water pressure, and electrical supply
available in the installation location. If the boiler does not match each of these ratings, do not install the boiler. If conversion to Propane
Gas is required, the included gas conversion kit must be used. Refer to“12.1 Gas Conversion” on page 97 for details.
WARNING
Ensure that the gas type and power source specifications match what is listed on the rating plate. Using a different gas type will cause
abnormal combustion and boiler malfunction. Using abnormally high or low AC voltage may cause abnormal operation, and may reduce
the life expectancy of the product.
This appliance complies with the requirements of SCAQMD Rule 1146.2 for NOx emissions of 14 ng/J or 20 ppm at 3% O
2
.

13Installing the Boiler
2.1 Removing the Wood Pallet from the
boiler
After you open the box, remove the wood pallet and the pallet
brackets and then install the boiler.
CAUTION
After opening the box, check the items of the upper container.
Removing the Wood Pallet and the Pallet Brackets
1. Loosen the six screws that secure the wood pallet on the
bottom left and right sides of the boiler.
2. Remove the wood pallet, the upper container, and the lower
pad.
Upper
Container
Lower Pad
Wood Pallet
3. Loosen the four screws that secure the pallet brackets.
CAUTION
Keep the removed screws in a safe place. The screws will be
used again in Step 5.
2. Installing the Boiler

14 Installing the Boiler
2.2 Choosing an Installation Location
When choosing an installation location, you must ensure that the
location provides adequate clearance for the boiler, adequate
venting and drainage options, and sufficient access to gas, water,
and electrical supplies. Carefully consider the following factors
when choosing an installation location:
Compliance Requirements
●Local, state, provincial, and national codes, laws, regulations, and
ordinances.
●National Fuel Gas Code, ANSI Z223.1-latest edition.
●Standard for Controls and Safety Devices for Automatically Fired
Boilers, ANSI/ASME CSD-1, when required.
●National Electrical Code.
●For Canada only: B149.1 Installation Code, CSA C22.1 Canadian
Electrical Code Part 1, CSA-B214-12 Installation code for
hydronic heating systems, and any local codes.
Access to Utilities
●Water – the installation location should be near where the
domestic water supply enters the building.
●Gas – the installation location should be near where the gas
supply enters the building.
●Electricity – the installation location should be near where the
electrical supply enters the building.
Humidity and Contact with Water
When installing the boiler, avoid places with excessive humidity.
The boiler has electric gas ignition components. Water spray or
droppings can get inside the boiler and damage the ignition
system. The boiler must be installed in a way to ensure that the gas
ignition system components are protected from water (dripping,
spraying, rain, etc.) during operation and service.
4. Remove the two pallet brackets.
5. Tighten the four screws into the original position.
CAUTION
Ensure that the screws have been retightened. If you do not
retighten the screws, the boiler’s airtightness will decrease.

15Installing the Boiler
Adequate Drainage
The boiler produces a significant amount of condensate during
operation. The boiler should be located near a suitable drain and
where damage from a possible leak will be minimal. Installing
the boiler in a location without a drain will void the warranty and
Navien will not be responsible for water damages that occur as a
result. For more information about condensate drainage, refer to
“3.2 Connecting the Condensate Drain”on page 24.
The boiler must be located in an area where leakage of the unit or
connections will not result in damage to the area adjacent to the
appliance or to lower floors of the structure. When such locations
cannot be found, installation of an adequately drained drain pan
under the boiler is highly recommended. When installing the drain
pan, ensure that the installation does not restrict combustion air
flow.
Adequate Venting and Ventilation
Select a location that requires minimal venting. Consider venting
restrictions caused by windows, doors, air intakes, gas meters,
foliage, and other buildings. For more information about venting,
refer to“5. Venting the Boiler” on page 51.
To ensure adequate venting and ventilation, follow these
guidelines:
●Maintain proper clearances from any openings in the building.
●Install the boiler with a minimum clearance of 12 in (300 mm)
above the level required by local codes and weather conditions.
●Maintain a minimum clearance of 4 ft (1.2 m) from heating and
cooling vents.
●Do not enclose the vent termination.
●Install the exhaust vent in an area that is free from any
obstructions, where the exhaust will not accumulate.
●Do not install the boiler where moisture from the exhaust may
discolor or damage walls.
●Do not install the boiler in bathrooms, bedrooms, or any other
occupied rooms that are normally kept closed or not adequately
ventilated.
Adequate Installation Clearances
CAUTION
Do not install the boiler on carpeting.
Install the boiler in an area that allows for service and maintenance
access to utility connections, piping, filters, and traps. Based on
the installation location, ensure that the following clearances are
maintained:
Top
Back
Side
Bottom
Front
Side
Clearance from: Indoor Installation
Top 9 in (229 mm) minimum
Back 0.5 in (13 mm) minimum
Front 4 in (100 mm) minimum
Sides 3 in (76 mm) minimum
Bottom 12 in (300 mm) minimum

16 Installing the Boiler
WARNING
●The boiler is heavy. Always lift the unit with assistance. Be
careful not to drop the boiler while lifting or handling it to
avoid bodily injury or damage to the unit.
●Do not rest the boiler on the bottom end after removing it
from the shipping carton. Doing so may result in excessive
pressure on protruding pipes and resulting in product
damage. If you must put the boiler down, lay it on its back or
put it inside the protective shipping base.
CAUTION
Do not mount the boiler to dry wall that has not been
reinforced.
CAUTION
It is necessary to leave clearance for service access.
Clean, Debris and Chemical-free Combustion Air
●Do not install the boiler in areas where dust and debris may
accumulate or where hair sprays, spray detergents, chlorine, or
similar chemicals are used.
●Do not install the boiler in areas where gasoline or other
flammables are used or stored.
●Ensure that combustible materials are stored away from the
boiler and that hanging laundry or similar items do not obstruct
access to the boiler or its venting.
2.3 Mounting the Boiler to the Wall
Navien NFB boilers come with an upper mounting bracket that
is pre-drilled at 16 in (400 mm) on center for easy installation on
standard wall studs. If the strength of the wall is insufficient or if
the framing is non-standard or uneven, reinforce the area before
installing the boiler. Avoid installation on common walls as the unit
will make some operational noises while it is running.
To mount the boiler to the wall:
1. Affix the bracket securely to the wall and ensure that it is level
and that it can support the weight of the boiler.
2. Align the grooves on the back of the boiler with the tongues
on the mounting bracket and hang the boiler on the bracket.
When mounted with the mounting bracket, the boiler will
have a
5
/
8
in (16 mm) clearance from the back of the wall.
3. Install two set screws through the bracket at the bottom of the
boiler to secure the boiler on the wall.

17Installing the System Piping
Prior to connecting plumbing to the boiler, flush the entire system
to ensure it is free of sediment, flux, solder, scale, debris or other
impurities that may be harmful to the system and boiler. During
the assembly of the heating system, it is important to keep the
inside of the piping free of any debris including construction dust,
copper burr, sand and dirt.
For retrofits, all system piping including radiators, must be cleaned
of all build-up including sludge and scale. All systems, old and new,
must be cleaned to remove flux, grease and carbon residue. Navien
recommends cleaning the boiler system with cleaning products
specially formulated for boiler systems. For retrofit applications
with heavy limescale and sludge deposits, a heavier duty cleaner
may be required. For information on performing the cleaning,
follow the instructions included with the boiler system cleaner
products.
WARNING
Failure to rid the heating system of the contaminants listed
above will void your warranty and may result in premature
heat exchanger failure and property damage.
3.1 Installing a Space Heating System
The fire-tube type heat exchanger of the Navien NFB boiler is
designed to attain the highest level of heat transfer in a compact
design. To accomplish this, the heated gas flows through a series
of small-diameter tubes, maximizing the heat transfer area. To
maintain the efficient and reliable operation of the heat exchanger,
and to avoid heat exchanger failure, it is critical to ensure the rules
and guidelines in this section are followed.
CAUTION
Failure to follow the instructions provided in this section will
void the warranty and may result in property damage, fire,
serious injury or death.
3. Installing the System Piping
3.1.1 Guidelines for a Space Heating Installation
Read and follow the guidelines listed below to ensure safe and
proper installation of a boiler heating system.
Freeze Protection for a Space Heating System
●Freeze protection products may be used for the space heating
system. Freeze protection for new or existing systems requires
specially formulated glycol, which contains inhibitors to prevent
the glycol from attacking the metallic system components.
●Before using freeze protection products, ensure that system
fluid contains proper glycol concentration and the inhibitor level
is appropriate. Navien recommends against exceeding a 50%
concentration of glycol.
●When using the freeze protection products, the system must
be tested at least once a year, and as recommended by the
manufacturer of the glycol solution.
●When using the freeze protection products, allowance should be
made for expansion of the glycol solution.
●Freeze damage is not covered by the warranty.
●The use of glycol results in a greater head loss due to its higher
viscosity compared to water. See page 23 for additional
information regarding the use of glycol.
WARNING
For systems requiring freeze protection, use only inhibited
propylene glycol, specially formulated for hydronic heating
systems; use of other types of antifreeze may be harmful to the
system and will void the warranty.
System Pressure
●The Navien NFB boiler is intended solely for use in pressurized
closed loop heating systems operating with 12-80 psi water
pressure at the boiler outlet. To obtain the minimum system
design pressure, follow the piping diagrams illustrated in this
section.
●The Navien NFB boiler’s space heating system is not approved
for operation in an“open system”, thus it cannot be used for
direct potable water heating or to process heating of any kind.

18 Installing the System Piping
Oxygen Elimination
This boiler may only be installed in a pressurized closed-loop
heating system, free of air (oxygen) and other impurities. To avoid
the presence of oxygen, ensure all of the air is removed from
the system during commissioning via strategically placed and
adequately sized air removal devices, located throughout the
heating system.
Note See the examples of system application at the end of
this section detailing the installation location of the air
removal device, in case an additional air removal device
is required for a specific application.
WARNING
●Immediately repair any leaks in the system plumbing to
avoid adding make-up water. Make-up water adds oxygen
and minerals to the system that may lead to heat exchanger
failure.
●Failure to follow these instructions may result in poor
performance, unnecessary wear of system components and
premature failure.
CAUTION
Do not solder piping directly onto the water connections, as
the the heat may cause damage to internal components.Use
threaded water connections only.
3.1.2 Essential Elements in a Space Heating System
Air Vent
The Navien NFB boiler comes with an air vent and an adapter
bushing that must be connected to the air vent connection. The
vent efficiently removes the air from the space heating system.
The following figure illustrates an example of a typical air vent
installation.
Return
Air Vent
Bushing
Air Vent
Supply
Circulation
Pump
Also, an external LWCO and a pressure relief valve can be installed
at the air vent connection. Refer to“Low Water Cut Off (LWCO)
Device”on page 19 and “Pressure Relief Valve” on page 20.
Note Before installing the vent line and any vent fittings,
you must be familiar with the LWCO and pressure relief
valve installation guidelines.

19Installing the System Piping
Low Water Cut Off (LWCO) Device
Internal LWCO
The Navien NFB boiler is equipped with a factory-installed,
pressure-sensing type low water cutoff (LWCO) device. The
minimum operation pressure for this device is 6 psi by default.
Note When the built-in water pressure sensor detects
insufficient water level in the boiler, error code E302 is
displayed on the front panel requiring a manual boiler
reset.
Refer to local codes to determine if a LWCO device is required
for your system and ensure that the built-in device meets the
requirements.
External LWCO
If local codes require a separate LWCO device, install one
separately. Combining the LWCO with the air vent simplifies the
installation. The following illustration shows an example of a
separately installed external LWCO.
6" (152 mm) min.
External
LWCO
The external LWCO must be installed at least 6 in (152 mm) above
the top of the heat exchanger. Refer to“3.6.2 Wiring Diagram -
External LWCO”on page 34 for typical wiring connections of the
LWCO to the boiler PCB.
Backflow Preventer
Install a backflow preventer valve in the make-up water supply to
the unit as required by local codes.
Expansion Tank
An expansion tank must be installed in the space heating piping
to prevent excessive pressure from building in the system. See the
examples of system application at the end of this section for the
installation location. Refer to the expansion tank manufacturer’s
instructions for additional details.
Follow the guidelines below when installing an expansion tank.
●Connect an air separator to the expansion tank only if the air
separator is located on the suction side of the system pump.
●Install the make-up water connection at the same location as
the expansion tank’s connection to the system.
●When replacing an expansion tank, consult the expansion tank
manufacturer’s literature for proper sizing.
●For diaphragm expansion tanks, always install an automatic air
vent on the top side of the air separator to remove residual air
from the system.
Isolation Valves and Unions
●Full port ball valves are required for the boiler system. Failure
to use full port ball valves could result in a restricted flow rate
through the boiler.
●Check valves are recommended for installation. Failure to install
check valves could result in a reverse flow condition during
pump(s) off cycle.
●Unions are recommended for unit serviceability.

20 Installing the System Piping
Pressure Relief Valve
To complete the space heating system installation, you must install
a
3
/
4
in, 80 psi (max.) pressure relief valve on the space heating hot
water outlet. An ASME approved HV pressure relief valve for space
heating system is supplied with the boiler.
WARNING
●Installing the pressure relief valve improperly may result
in property damage, personal injury, or death. Follow all
instructions and guidelines when installing the pressure
relief valve. The valve should be installed only by a licensed
professional.
●The pressure relief valve must be installed at the boiler
outlet and in the vertical position, as shown in this section,
with the drain pipe outlet exiting the side of the pressure
relief valve horizontally and elbowing down.
CAUTION
Install the pressure relief valve as close to the boiler as possible.
No other valve should be installed between the pressure relief
valve and boiler.
When installing the pressure relief valve, follow these guidelines:
●Ensure that the valve’s discharge capacity is equal to or greater
than the maximum pressure rating of the boiler’s space heating
system.
●Ensure that the maximum BTU/H rating on the pressure relief
valve is equal to or greater than the maximum input BTU/H
rating of the boiler.
●Direct the discharge piping of the pressure relief valve so that
hot water does not splash on the operator or equipment.
●Attach the discharge line to the pressure relief valve and install
the end of the line within 6-12 in (150-300 mm) above the floor.
●Ensure that the discharge line provides complete and
unobstructed drainage. Do not install a reducing coupling or
other restrictions on the discharge line.
If the relief valve discharges periodically, this may be caused by
thermal expansion when the expansion tank is full or undersized.
Do not plug the relief valve.
There are two pressure relief valve installation methods. In Method
1, the pressure relief valve is installed on top of the boiler and in
Method 2 it is installed in a manifold system.
If the installed system does not have a Navien manifold system,
install a pressure relief valve with the air vent. The illustration
below shows an example of a pressure relief valve installed with
the air vent.
Method 1.
External LWCO
Air Vent
Pressure Relief
Valve
Refer to the following illustration for a pressure relief valve
installation in the space heating system. A Watts 174A pressure
relief valve (
3
/
4
in, HV, Max 30 psi) is provided with the boiler.
Pressure Relief Valve
Method 2.
Note Depending on the installation conditions, pressure
relief valves (not included and for separate purchase) of
up to 80 psi can be used.
Refer to“11.4.3 Setting the Operation Parameters - 16.
High Sys Pressure”on page 92 to adjust the upper
limit pressure lockout setting when using a higher
rated valve.
CAUTION
All separately purchased pressure relief valves must be ASME
certified.